1. Field of the Invention
The present invention relates generally to image sensors, and more specifically to a CMOS-based image sensor.
2. Description of the Related Art
CMOS (complementary metal oxide semiconductor) based image sensors are characterized by low power consumption, operability on a single voltage source (5 or 3.3 volts) and amenability to integration with peripheral circuitry on a common chip, as described in a paper titled xe2x80x9cA 128xc3x97128 CMOS Active Pixel Image Sensor for Highly Integrated Imaging Systemsxe2x80x9d, Sunetra K. Mendis et al., IEDM 93, pages 583 to 586.
However, one disadvantage of the current CMOS-based image sensor is that the output voltage it generates is not of sufficient value.
It is therefore an object of the present invention to provide a CMOS-based image sensor capable of producing a high voltage output.
The present invention is based on a recognition that there is a stray capacitance between a grounded conductive light shield covering portions of the image sensor other than photosensitive areas and a conductor extending parallel with the light shield. It is found that the presence of this stray capacitance is the source of the stated problem.
The problem is solved by segmenting the conventional light shield into a plurality of unit light shields respectively corresponding to unit cells of the image sensor and biasing each unit light shield at a potential which varies with the output voltage of the corresponding unit cell.
According to the present invention, there is provided an image sensor comprising a plurality of light shields of conductive material, the light shields being formed with respective openings. A matrix array of unit cells are formed on a semiconductor body so that they respectively correspond to the light shields. Each of the unit cells includes a photosensitive region for receiving light through the opening of the corresponding light shield for producing photo-generated electrons, a floating diffusion region, a transfer gate for transferring the photo-generated electrons from the photosensitive region to the floating diffusion region, and an amplifying transistor having a controlling terminal connected to the floating diffusion region for amplifying a potential developed in the floating diffusion region and a controlled terminal connected to the corresponding light shield, so that the light shield is ii biased at a potential that substantially varies with the potential of the floating diffusion region.